Cisco Cisco IOS Software Release 12.4(2)XB6

For general information on caveats and the bug toolkit, see About Cisco IOS Release Notes located at 

This section contains the following caveat information:

There are no open caveats in this release.

CSCsv04836

Multiple Cisco products are affected by denial of service (DoS) vulnerabilities that manipulate the 
state of Transmission Control Protocol (TCP) connections. By manipulating the state of a TCP 
connection, an attacker could force the TCP connection to remain in a long-lived state, possibly 
indefinitely. If enough TCP connections are forced into a long-lived or indefinite state, resources on 
a system under attack may be consumed, preventing new TCP connections from being accepted. In 
some cases, a system reboot may be necessary to recover normal system operation. To exploit these 
vulnerabilities, an attacker must be able to complete a TCP three-way handshake with a vulnerable 
system. 

In addition to these vulnerabilities, Cisco Nexus 5000 devices contain a TCP DoS vulnerability that 
may result in a system crash. This additional vulnerability was found as a result of testing the TCP 
state manipulation vulnerabilities. 

Cisco has released free software updates for download from the Cisco website that address these 
vulnerabilities. Workarounds that mitigate these vulnerabilities are available. 

This advisory is posted at